A display device using a liquid crystal compound (in the present application, a term “liquid crystal compound” is used as a generic term for a compound having a liquid crystal phase, and a compound having no liquid crystal phase but being useful as a component of a liquid crystal composition) has widely been used for a display for a watch, a calculator, a word processor or the like. The display devices utilize optical anisotropy, dielectric anisotropy and so forth of the liquid crystal compound.
The liquid crystal display device typified by a liquid crystal display panel, a liquid crystal display module and so forth utilizes optical anisotropy, dielectric anisotropy and so forth of the liquid crystal compound. As an operating mode of the liquid crystal display device, a variety of modes are known, such as a phase change (PC) mode, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, a bistable twisted nematic (BTN) mode, an electrically controlled birefringence (ECB) mode, an optically compensated bend (OCB) mode, an in-plane switching (IPS) mode, a vertical alignment (VA) mode and a polymer sustained alignment (PSA) mode.
The liquid crystal display devices include a liquid crystal composition having suitable physical properties. In order to improve characteristics of the liquid crystal display device, the liquid crystal composition preferable has suitable physical properties. As general physical properties necessary for the liquid crystal compound being a component of the liquid crystal composition, the liquid crystal composition is required to have characteristics shown in (1) to (6), more specifically.
(1) chemical stability and physical stability,
(2) a high clearing point (clearing point: transition temperature between a liquid crystal phase and an isotropic phase);
(3) a low minimum temperature of the liquid crystal phase (a nematic phase, a smectic phase or the like), in particular, a low minimum temperature of the nematic phase;
(4) a suitable optical anisotropy;
(5) a suitable dielectric anisotropy, and
(6) an excellent compatibility with other liquid crystal compounds.
When a composition containing a compound having a chemical and physical stability as described in (1) is used for a liquid crystal display device, a voltage holding ratio can be increased. In particular, a high voltage holding ratio is required for a TFT active matrix liquid crystal display device that currently constitutes a mainstream.
In a composition containing a liquid crystal compound having a high clearing point or a low minimum temperature of the liquid crystal phase as described in (2) and (3), a temperature range of the nematic phase is wide, and therefore the device can be used in a wide temperature range.
Furthermore, in the case of a display device using a composition containing a compound having a suitable optical anisotropy as described in (4), contrast of the device can be improved. A device requires compositions having a small to large optical anisotropy depending on a design of the device. A technique has been recently studied for improving response speed by decreasing cell thickness, and accordingly a liquid crystal composition having a large optical anisotropy is also required.
As is well known, threshold voltage (Vth) is represented according to the following equation (H. J. Deuling et al., Mol. Cryst. Liq. Cryst., 27 (1975) 81):Vth=π(K/∈0Δ∈)1/2 wherein, in the equation, K represents an elastic constant and ∈0 represents a dielectric constant of vacuum. As is known from the equation, two ways are conceivable in order to decrease Vth: either increasing a value of dielectric anisotropy (Δ∈) or decreasing K. However, according to the present technology, actual control of K is still difficult. Therefore, under the present situation, a liquid crystal material having a large Δ∈ is ordinarily used to respond to the requirement. Under such circumstances, a development has been actively made for a liquid crystal compound having a suitable dielectric anisotropy, as described in (5), particularly, a liquid crystal compound having a large dielectric anisotropy.
In a passive matrix liquid crystal display device, the STN mode is most frequently applied. In the STN mode, a compound having a cyano group is mainly used as a liquid crystal compound having a large dielectric anisotropy. However, the compound having the cyano group has a low voltage holding ratio, and thus the compound having the cyano group is not currently used in the TFT active matrix liquid crystal display device in the mainstream, and a fluorine liquid crystal compound is used.
The liquid crystal compound is generally used in the form of a composition prepared by mixing the compound with many other liquid crystal compounds in order to develop characteristics that are difficult to be attained by a single compound. Accordingly, a liquid crystal compound used for the display device has an excellent compatibility with other liquid crystal compounds and so forth as described in (6). Moreover, the display device may be occasionally used in a wide temperature range including a freezing point, and therefore a compound exhibiting an excellent compatibility from a low temperature range may be occasionally preferred.
Specific examples of a liquid crystal compound having a 2,6,7-trioxabicyclo[2.2.2]octane ring are disclosed in Patent literature No. 1, Patent literature No. 2 and Non-patent literature No. 1. The literatures show compound (a), (b) or the like. However, compound (a) has a small dielectric anisotropy and is quite difficult to be used for the purpose of decreasing driving voltage of the liquid crystal display device, or the like. Moreover, compound (b) has a cyano group, and therefore has a low voltage holding ratio. Further, dipoles of the cyano group and the 2,6,7-trioxabicyclo[2.2.2]octane ring are reversely directed, and therefore the dielectric anisotropy does not increase.

Patent literatures No. 3 to No. 7 describe a compound having a 2,6,7-trioxabicyclo[2.2.2]octane ring. However, all the literatures relate to an insecticide or innovative drug development, and describe nothing on an application as a liquid crystal material.